1. Field of the Invention
"Imager devices", or "image sensors" as used herein shall refer to devices which are sensitive to electromagnetic radiation and which, in conjunction with electrical means, provide electrical signals indicating presence of such radiation. Their principal application may well be found in providing electrical signals directly relatable to visual images. This invention relates to imager devices and more particularly to a multi-element imager device (MEID) comprised of a plurality of imaging charge coupled devices (ICCD's) mounted to and supported by a stratum of material with selectable coefficient of optical transmissivity through which, target, or image, energy passes to activate sensing elements of such ICCD's. Such MEID's find application as focal planes in high resolution television cameras or surveillance systems in places inaccessible to human operators (e.g. space satellites or nuclear reactors). High sensitivity of the component ICCD's makes the MEID especially valuable where light intensity from the target source is low. Use of such radiation sensitive devices with the particular transmissivity of a selected supporting stratum allows accurate reproduction of images in the sensed wavelength region of the spectrum while providing protection of the devices from X-rays, lasers, and harmful radiation to which the stratum is opaque.
This supporting stratum may also carry signal, power and timing buses to enable close packing of ICCD's in a reliable, readily producible configuration minimizing "dead" or unused space in the sensing plane. "Sensing plane", as used in this disclosure, shall mean that plane formed by the active surface of the radiation sensitive device when mounted to its supporting stratum.
2. Description of the Prior Art
Image sensor chips are manufactured in a variety of configurations, all directed toward the same general purpose, so that when "ICCD" or "radiation sensitive device" is used in this disclosure, it shall be understood that any device capable of presenting electrical signals corresponding to radiation in the sensing plane is comprehended. Charge coupled devices are in widespread use throughout modern industry and the within invention was reduced to practice utilizing such units. Any device, however, providing electrical signals corresponding to visual or optical images over its sensing surface is equivalent to the "ICCD's" referred to herein. ICCD's may also be useful for sensing and converting radiation outside the visible spectrum.
Image sensors are utilized in television cameras and other electro-optic systems to convert a visual presentation on the sensor's surface into a serial pulse train of electrical signals. The pulse train is then processed and conditioned for whatever purpose is to be made of information contained therein including remote reproduction of the sensed image. When the within MEID is used for image reproduction as part of an electro-optical depiction system, it will be placed in the focal plane of an optics system comprised of lenses, collimators, etc. Multiplexers associated with the component ICCD's convert the image sensed in such a focal plane to a train of pulses. The amplitude of which pulses is directly proportional to the intensity of illumination occurring at the photosites or pixels of the sensors.
Such ICCD's are not new in the art of photo optics and they are available, commercially, in a variety of configurations ranging from a few millimeters in linear dimensions to fractional inches. Typical sensors are those of the Fairchild Semiconductor Company known as Model CCD211, and of Radio Corporation of America (RCA), known as Model SID51232.
The former device contains 46,360 photosites or pixels on approximately 23 micron centers, each pixel being approximately 18.times.14 microns in dimension over the 4.4 by 5.7 millimeter sensing surface.
The RCA device has, roughly, 163,840 pixels over a sensing surface of 7.3 by 9.7 millimeters.
Most such image sensors are charge coupled devices (CCD's) and are manufactured integral with a multiplexer unit. The pixels are sequentially sampled at frame rates of from 50 to 100 per second by the multiplexer whose output is coupled directly to additional signal processing circuitry. Processing of multiplexer outputs can be accomplished in a variety of modes. In a MEID of several inches on a side, for example, perimeter chips may well serve to trigger operation of close packed arrays internal to the plane as a target image is sensed passing into the plane area. Composite images of focal plane targets may be reconstructed by sequentially sampling individual lines of component chips so that, instead of sampling all pixels of a given chip, certain sections of given chips might be specially processed to provide greater detail and resolution of target data. Processing of chip data is limited only by the imagination of designers and sequences of operation may well be programmed into electronic processors.
Each pixel produces an output signal proportional to the intensity of light (viz. electromagnetic energy) impingent thereon. Optical focusing and collimating equipments condition the energy in the focal plane so that wide aperture lens scanners providing maximum target energy gathering may be integrated with the sensitive ICCD's. By utilizing a plurality of such ICCD's and distributing target energy among them, a manifold increase in resolution of target detail results. While a single chip may well produce the target image upon proper preconditioning of light in the optics for illumination of chip pixels, such use provides excess energy for certain pixels on which detail from two adjacent target points of interest may fall. A large scale focal plane segregates a small portion of the target image for detail processing by a given chip in the focal plane array and provides a high resolution of detail in that portion.
CCD's are highly reliable and useable with low intensity light source targets because of inherent high sensitivity of the individual pixels. Integration of a number of such units into a given MEID combines the good features of sensitivity and reliability with high resolution of target data deriving from each unit's use for portraying a small part of the target.
While the above individual chips function well in their design modes, their sensing surfaces are often too small to provide resolution required for detail analysis of target images. Problems met in arranging these individual ICCD's into large scale focal planes are solved by the materials and processes of this invention.
The primary problem met in developing large-area focal planes utilizing conventional ICCD sensor chips is connection of signal and timing leads between signal processing electronics and the two dimensional detector arrays proper. Existing technology utilizes sandwiches of printed circuit boards with ICCD's place upon the edges of the boards so that wiring between a given chip and its electronics is coupled over the edge of the board to components mounted on the board face. This arrangement requires precise mechanical operations for soldering, optical alinement, shock mounting, vibration isolation, etc., and results in an unacceptably low yield of operational systems. Limitations inherent in the design preclude close packing of the boards and result in unacceptably limited array density as well. Substantial "dead space" in the focal plane is a natural consequence of such packing since no photosites, viz. pixels, occur in those areas of the plane used for isolating the boards, mechanically and electrically, from each other. This, of course, mandates incomplete sensing of the total scene of interest. The MEID described here minimizes this defect.
The within invention overcomes lead connection and mechanical spacing problems in a novel fashion and allows imager chips to be arrayed in extended focal planes of dimensions measured in inches rather than the millimeters of the chips proper.
U.S. patents, including 3,117,260, to Fairchild Semiconductor Co., define and explain the individual imager chips such as are used with this invention. Other U.S. patents such as No. 3,842,492 Method of Providing Conductor Leads for a Semiconductor Body, U.S. Pat. Nos. 3,846,823 Semiconductor Assembly, and 3,821,785 Semiconductor Structure with Bumps, are applicable to parts of this invention. None, however, provides the means for integration of a number of ICCD imager chips into the large scale focal plane of this invention.
Throughout this disclosure, the term superstrate will refer to a body of transparent material interposed between the sensed target and the sensing surface of imager chips. Placed, as it is, between the target and sensing surface, its denomination as superstrate differentiates it from conventional support elements known as substrates.
In embodiments of this invention, an optically transparent superstrate, of selectable transmissivity, is utilized as mechanical support for an array of CCD imager chips. This superstrate similarly supports electrical buses and has electrically conducting standoff pads connected to these buses around its periphery. Each standoff is positioned to mate with electrically conductive pads of the individual CCD chips and provides interconnection of chips with system power, timing and output signals. Bonding pads are provided with braze material coating and, in one of the embodiments, the superstrate is fitted with braze or solder bumps in place of the above standoffs, so that interconnection is achieved by means of the solder bumps proper.
Each standoff and chip pad is brazed or soldered together utilizing conventional flip-chip bonding technology wherein controlled atmospheres and mechanical positioning apparatus allow all CCD chips to be brazed to the superstrate standoffs while they are being held to close tolerance coplanarity with each other. Alternate methods of mounting such radiation sensitive devices to said superstrates may of course be used. As used herein, "braze" or "solder" includes any of such alternate methods. "Braze" or "solder" materials shall also include the materials used in alternate methods.
One of the preferred embodiments of this invention utilizes optically transparent Indium Oxide buses to interconnect devices of the array. Bands of Indium Oxide, a few microns thick, are deposited, in conventional fashion, onto the superstrate in the desired pattern (see FIG. 6). These bands of Indium Oxide, in turn, may terminate in pins of multi-pin plug-in packages for convenience of mounting of individual focal planes (FIG. 3).
Electrical buses of the superstrate are designed to interconnect detector chips in parallel fashion for such functions as power and timing. Video output of each chip, however, is bussed to its own separate bonding pad for processing to system requirements (see FIG. 8).
Such a parallel electrical arrangement of buses to interconnect chips of the array eliminates a common single point failure of similar systems wherein open circuit of a critical bus line (timing, power, etc.) resulting from connector pin anomalies or physical damage to a bus line, disables the entire system. For the within device, such circuit defects are compensated by the opposite terminal segment of the bus line which then supports operation entirely on its own.
Where transparent buses are not used, target image integrity is preserved by using opaque bus material routed between chip pixel rows or around the periphery of the chip proper (see FIGS. 5 and 7). Such opaque buses may be as narrow as a few microns of plated conductor and as thin as one or two microns, deposited conventionally on the superstrate during fabrication thereof. It is understood, of course, that the conductors may be of semiconductor material such as doped Germanium, Silicon, Gallium Arsenide, etc. Thin layers of silicon dioxide or other insulative material may be deposited over the buses where desired.